In an electrical network powered by an alternating current generator, it is generally important that the frequency of the generator be held stable, for example, within a five percent range of a target frequency of 60 Hz. This stability requirement poses a problem when a twin-spool gas turbine engine is used to drive the electrical generator. (A twin-spool engine is one having a first, high pressure spool concerned with the generation of a high velocity gas stream, and thus the first spool is also referred to as a gas generator stage. A second, power spool is utilized in the extraction of mechanical energy from the gas stream by means of a power turbine. The mechanical energy extracted is utilized to drive the electrical generator.)
A problem arises when the electrical load on the electrical generator changes, as when a large electrical load is either connected to, or disconnected from, the network. The load change causes a change to occur in the mechanical energy demanded by the electrical generator, thus necessitating a change in the energy in the gas stream delivered by the gas generator.
In general, the energy delivered by the gas generator cannot change instantaneously because the energy contained in the gas stream is a function of the rotational speed of rotating components of the gas generator. These components need a finite time to change speed. Thus, a time lag occurs between the change in electrical load demanded and the change in gas generator speed in response. During this time lag the frequency of the electrical generator deviates from the target frequency. Many times, this deviation is too great in either magnitude, duration, or both, to be tolerated by the electrical network.
In addition, the exhaust gases produced by the gas generator are commonly recovered by a heat exchanger after these gases have delivered energy to the power turbine. It is known that a heat exchanger works with increased efficiency as the temperature gradient across it increases. However, it is also known that changing the speed of the gas generator in a twin-spool engine results in a change in the exhaust gas temperature, and that this change is lesser in magnitude than the corresponding change encountered in a single spool engine when the power output of the latter is changed. For example, as the power output decreases in a single spool engine in order to deliver proper energy to the power turbine, the exhaust gas temperature decreases significantly since airflow remains essentially constant while fuel flow decreases. In this example, the efficiency of the heat exchanger correspondingly decreases more than it would if a twin-spool engine were used. Therefore, even if the response time of the single-spool engine is satisfactory to maintain proper network frequency, decreased efficiency in heat recovery is encountered. Accordingly, while frequency may be maintained, the operating efficiency of the system is reduced.
Moreover, the power delivered in a single engine of nonvariable geometry drops approximately as the fourth power of speed. Accordingly, a deceleration resulting from a load increase can easily slow the engine to a point from which recovery is impossible because of the greatly reduced power output at the lower speed. Typically, an electrical generation utilizes multiple engines. In a system using multiple single spool engines, the response to an overload condition is to disconnect equipment from the generators (i.e., to shed load) rapidly and, ideally, within a very short time such as 30 milliseconds. Given that it is difficult to detect an overload and then to shed load within such a short time, the occurrence of the overload can cause the engines to slow to an irrecoverable operating point.